Processes for the preparation of highly-branched polymers based on C3-C8 ethylenically unsaturated mono- or dicarboxylic acid are limited by their process parameters. Heat removal represents a considerable security and process control aspect, that strongly influences product properties. Another issue is that the manufacture of highly-branched polymers based on C3-C8 ethylenically unsaturated mono- or dicarboxylic acid at higher temperatures leads to a corrosion of the reaction tank and needs instruments that can withstand high pressures correlated to these high temperatures. For this reason longer reaction times are required, usually several hours. Therefore polymers obtained in semi-batch processes, which are characterized by limited process parameters, are restricted in their structural variations.
In order to obtain highly branched polymers based on C3-C8 ethylenically unsaturated mono- or dicarboxylic acids several additives and/or additional steps are usually applied. H. Mori et al. in Macromolecules 2002, 35, 9270-9281, describe the synthesis of highly branched polyacrylic acid via ATRP polymerization of tert-butyl acrylate and a brominated monomer followed by hydrolysis. These processes include expensive and potentially hazardous chemicals and complex procedures.
A way to control the molecular weight is to prepare the polymers by means of controlled radical polymerization in the presence of a chain transfer agent. For lower molar masses very high concentrations of the chain transfer agent are necessary, often in the range >10 weight %. The obtained polymers comprise then an undesired proportion of organic or inorganic components which are noticeable by an intense odour or colour or are undesired from a material point of view, for example because they are toxic or harmful to health, or else because they are disruptive in the intended application.
WO 2009/133186 A1 reads on a continuous process for the preparation of a polymer by free-radical polymerization in the presence of a chain transfer agent wherein the polymerization preferably takes place in a microstructured reaction zone. The polymerization of ethylenically unsaturated carboxylic acids such as acrylic acid is described and may be carried out at a temperature up to 200° C., preferably 60° C. to 110° C.
WO 2011/054789 A1 relates to a process for producing aqueous solutions of homo- or copolymers of acrylic acid in the presence of at least one water-soluble chain transfer agent at a temperature of up to 200° C., preferably 80° C. to 120° C. The low-molecular side-products resulting from the polymerization are later on eliminated by means of physical or physico-chemical separating methods.
Loiseau et al. in Macromolecules, 2003, 36, 3066-3077 describes the synthesis of polyacrylic acid through controlled radical polymerization. The synthesis of polyacrylic acid was carried out in a semi-batch process in the presence of a reversible addition-fragmentation chain transfer (RAFT) agent. The polymer thus obtained displayed a molecular weight dispersity index lower than 2 and a degree of branching lower than 2 mol %.
Couvreur et al. discloses in Macromolecules, 2003, 36, 8260-8267 a semi-batch process for the polymerisation of acrylic acid in the presence of nitroxide. Polyacrylic acid with a narrow molecular weight dispersity index and a degree of branching up to 6.5 mol % were synthesized.